Effect of irrigation on the severity of charcoal rot and yield on selected drought tolerant soybean genotypes
Alemu Mengistu: USDA ARS
<div>Charcoal rot, (caused by <em>Macrophomina phaseolina</em>), has historically resulted in severe yield losses of soybean [<em>Glycine max </em>(L.) Merr.] in the U. S. and around the world. Identification of resistance and yield measurements could be complicated because of the confounding effect of varying drought stress levels from year to year. The objectives were to determine the severity of charcoal rot (CR) in the putative drought tolerant genotypes and to determine the over-all effect of CR on yield in irrigated and non-irrigated environments. Colony Forming Units (CFUs) at reproductive stage R7 were used to assess disease severity and classify each genotype’s response to CR. A stress tolerance index (STI) was used to determine the relative impact of soil moisture stress (i.e. no irrigation) on the 13 genotypes. Over the three years (2011, 2012 and 2013) in both irrigated and non-irrigated environments, five genotypes were consistently rated as moderately resistant to CR and three genotypes were consistently rated as susceptible to CR, whereas the responses of the remaining five genotype’s varied. Averaged over the three years, there was a wide range (0.36 to 1.09) of STI values among genotypes. Even though there was a consistent trend, there was a very weak relationship between STI and CFU’s at R7. Regression analysis indicated that as CFUs at R7 increased, seed yield decreased, although the relationship was not significant in every year and irrigation environment. Nonetheless, across all years and irrigation environments, a pooled (global) slope indicated a yield loss of 11.5 kg ha<sup>-1</sup> for every 1,000 CFUs at R7. These data indicated that as CFUs at R7 increased, seed yield decreased in both irrigated and non-irrigated environments suggesting that, it may be necessary to select for resistance to both traits using environments where both soil moisture stress and CR are high.</div>
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